The present invention relates to a semiconductor memory device having multiple ports, and also to additional functions in the same. More particularly, it relates to techniques which are effective when applied to, for example, a buffer memory for communications data.
In communications, the quantity of pixel unit image information is considered large when compared with code information. It is therefore practiced in, for example, facsimile transmission that the quantity of information transmitted is compressed which shortens the data transmission time. A method for data compression is a one dimensional encoding method. This method encodes the information in blocks of pixels specified by binary information.
Where the data compression is done after temporarily storing the information to-be-encoded in a data buffer, the data buffer is accessed to read out the data and an encoding process for the compression is executed for the readout data through software or the like. This holds true also where the compressed data is expanded back into the original data. In other words, a conventional data buffer is not furnished with the functions of compressing/expanding data.
By the way, an example of publications stating the one-dimensional encoding method is "Microcomputer Handbook", pp. 1019-1020, issued by The Ohm-Sha, Ltd. on Dec. 25, 1985.
Besides, inventions for compressing data constructed on the same chip as that of a memory are disclosed in Japanese Patent Application Laid-open No. 63-183699 laid open on Jul. 29, 1988 in Japan and Japanese Patent Application Laid-open No. 63-204594 laid open on Aug. 24, 1988.
When the data buffer memory is not furnished with the data compressing/expanding functions as in the prior art, the temporary storage of the data and the process for compressing/expanding the data are inevitably separated, so that the data must be externally transferred between the data buffer memory and a compressing/expanding circuit. Moreover, when the compressing/expanding process is executed through the software, the burden on the processor is increased. The long time expended on the compressing/expanding process for the data temporarily stored, lowers the throughput of a system.
An object of the present invention is to provide a semiconductor memory device which can store data temporarily and which can execute processes for compressing/expanding data.
The above and other objects and novel features of the present invention will become apparent from the description of this specification when read in conjunction with the accompanying drawings.